disable_extension() made an assumption that it should always send an

extension_disabled() signal, and that it always had an extension

object to pass in.
If the extension being disabled was never enabled but had a load error,

this assumption would break. This is a pretty obscure case, but one that

was hit with some test work.
We now set extension to None in this case, and only emit the signal

if not None.

Unit tests pass.
Verified this fixed the issue I was hitting in upcoming test changes.